The natural sense of hearing in human beings involves the use of hair cells in the cochlea that convert or transduce acoustic signals into auditory nerve impulses. Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Some types of conductive hearing loss occur when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded. These sound pathways may be impeded, for example, by damage to the auditory ossicles. Conductive hearing loss may often be overcome through the use of conventional hearing aids that amplify sound so that acoustic signals can reach the hair cells within the cochlea. Some types of conductive hearing loss may also be treated by surgical procedures.
Sensorineural hearing loss, on the other hand, is caused by the absence or destruction of the hair cells in the cochlea, which are needed to transduce acoustic signals into auditory nerve impulses. People who suffer from severe to profound sensorineural hearing loss may be unable to derive significant benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus. This is because the mechanism for transducing sound energy into auditory nerve impulses has been damaged. Thus, in the absence of properly functioning hair cells, auditory nerve impulses cannot be generated directly from sounds.
To overcome sensorineural hearing loss, numerous cochlear implant systems—or cochlear prostheses—have been developed. Cochlear implant systems bypass the hair cells in the cochlea by presenting electrical stimulation directly to the auditory nerve fibers by way of an array of electrodes implanted within the cochlea. Direct stimulation of the auditory nerve fibers leads to the perception of sound in the brain and at least partial restoration of hearing function.
In a typical cochlear implant system, the electrical stimulation used to stimulate auditory nerve fibers is generated by one or more current sources included in a cochlear implant. For example, a pair of current sources (e.g., a positive current source and a negative current source) may be used to generate stimulation current (e.g., a biphasic stimulation pulse) that is applied to a particular electrode included in the cochlear implant system. Unfortunately, a current source included in a cochlear implant may not be precisely accurate due to variations in wafer process, layout, and design. In other words, the current source may not output the exact amount of current that it is commanded to output. A discrepancy in actual versus intended current output by a current source may result in various negative side effects including imbalanced biphasic stimulation pulses, a loss of compliance voltage to the current source, inaccuracies in the stimulation current provided to a patient, and/or unnecessary power consumption.